Removed unnecessary Secondary inheritance from RefundEscrow. (#1381)

(cherry picked from commit 308e5e9c)

contracts/payment/RefundEscrow.sol

 pragma solidity ^0.4.24;
 
 import "./ConditionalEscrow.sol";
-import "../ownership/Secondary.sol";
 
 
 /**
@@ -10,7 +9,7 @@ import "../ownership/Secondary.sol";
  * The primary account may close the deposit period, and allow for either withdrawal
  * by the beneficiary, or refunds to the depositors.
  */
-contract RefundEscrow is Secondary, ConditionalEscrow {
+contract RefundEscrow is ConditionalEscrow {
   enum State { Active, Refunding, Closed }
 
   event Closed();